The two methods currently in wide use which separate the produced metal from the byproduct salt and excess reductant are time-consuming and costly. For example, metal produced by the Kroll process (magnesium reduction) is often vacuum distilled. During vacuum distillation, the product of the reduction reaction, in the form of a regulus, is subjected to a vacuum heat treatment, in which the magnesium chloride and excess magnesium are evaporated from the product metal. This process operates at temperatures up to 1000.degree. C., and at vacuum levels down to 10 microns or lower. The final result is a mass of the product metal which has a porous structure. Essentially, the very fine particles of the product metal sinter together during the high temperature and vacuum conditions.
On the other hand, metal produced by the Hunter process is often leached in order to dissolve the sodium chloride and to hydrolyze the excess sodium. This is in preference to vacuum distillation, because sodium chloride is not as volatile as magnesium chloride, and thus is not as easily separated by the vacuum distillation method. In some cases, metal produced by the Kroll process is also leached, although an acid solution must be used to dissolve the excess magnesium.
In the case of vacuum distillation, there are several drawbacks. The vacuum distillation process requires up to five days to effect complete separation of the byproduct salt and excess reductant from the product metal. The product metal is recovered in a porous form which is referred to as "sponge". Sponge is often an undesirable form of the metal, because it has a large specific surface area when compared to consolidated, or homogeneous metal. This large surface area tends to absorb considerable amounts of oxygen from the atmosphere when the metal is exposed to air. As a matter of practice, distilled masses of sponge are crushed down to a small size so that they may be compacted into consumable electrodes for vacuum arc melting. The crushing operation creates a large amount of surface area, which leads to additional oxygen and nitrogen pickup from the atmosphere.
In the case of leaching, while not as much time is required to effect separation, the pickup of impurities is more of a problem. This is due to dissolved gases in leaching solutions, the evolution of gases during dissolution, and the exposure of the leached product to air. Similarly, the product metal is recovered in the form of sponge which is essentially less than desirable. In either case, after the vacuum distillation step or the leaching step, the sponge product metal is typically compacted to form an electrode for vacuum arc melting. In this step, the sponge is melted in a vacuum to form consolidated, homogeneous metal. Typical of the teachings of the prior art are U.S. Pat. Nos. 2,205,854; 2,482,127; and 4,242,136.